The polyester monofilaments have been widely utilized in not only the apparel field, but also the industrial material field. In particular, in the latter industrial material field, they are used as raw monofilaments, for example, for tire cords, ropes, nets, guts, tarpaulin, tents, screens, paragliders, sailcloth and the like. Physical properties required for these monofilaments have also become severe, and it has been urged to improve adhesiveness to rubber, fatigue resistance, dyeabiity, wear resistance, slub strength and the like.
In particular, the polyester monofilaments have recently taken the place of natural fibers such as silk and inorganic fibers such as stainless steel in the field of raw yarn for printing screen gauzes, because of their excellent dimensional stability.
However, in the recent printing field of electronic equipment such as printed wiring boards, the degree of integration increases more and more, and requirements for improvement in printing precision of screen gauzes, that is to say, requirements for high strength, high modulus and high mesh, have become severer therewith.
Accordingly, for the raw monofilaments, high-strength, high-modulus and finer ones have also been required.
In general, in order to increase the strength and modulus of the polyester monofilaments, what is necessary is just to perform hot drawing of spun filaments under a high draw ratio to highly orient and crystallize them.
However, in the subsequent process of screen gauze production, in order to comply with the requirement for the above-mentioned “high-mesh”, high-density fabric is woven, which causes raw filaments to receive severer repeating friction particularly with reeds. Accordingly, whisker-like or powdery scraping of filament surfaces frequently occurs to impair not only productivity but also the quality level of products.
Moreover, the more highly oriented and crystallized the raw filaments are, and further, the finer the filament diameter of the raw filaments is, the stronger the above-mentioned tendency becomes. As a result, the accumulation of the scraping of the filaments induces a standstill of a weaving machine, and further, the scraping of the filaments woven into the screen gauzes brings about print defects in precision printing.
As a measure for inhibiting this filament scraping in weaving, for example, it is proposed in patent document 1 (JP-A-55-16948) to use high-elongation raw filaments having a breaking elongation of 30 to 60% as warps. However, the modulus of the high-elongation raw filaments become low, to put it the other way around, which conflicts with the requirement for the high-strength and high-modulus screen gauge.
In order to obtain high-strength and high-modulus raw filament, high-ratio drawing is necessary as described above. It is said that this causes the orientation of a surface layer portion of the filament to become higher than that of a center portion, resulting in that a phenomenon in which the surface is partially scraped by friction is liable to occur.
As a countermeasure for this, it is also variously proposed that a melt of a surface layer portion of a filament is changed, thereby making increased strength and modulus compatible with filament scraping inhibition in weaving. For example, patent document 2 (JP-A-1-132829) proposes that a polyester is arranged in a core portion and a nylon is arranged in a sheath portion to form a sheath-core structure, thereby improving ability of inhibiting filament scraping, although it has high strength. However, in this case, there arises a disadvantage that the dimensional stability of a raw filament is impaired caused by high moisture absorption inherent to the nylon. Further, a raw filament structure is the sheath-core structure composed of the polyester and the nylon that have no compatibility with each other, so that it has a fear that separation is liable to occur in an interface of both polymers when repeated fatigue is applied in printing.
In order to solve this separation problem, patent document 3 (JP-A-2-289120) proposes to employ a sheath-core structure in which a polyester homopolymer having an intrinsic viscosity of 0.80 is arranged in a core portion and a polyethylene glycol-copolymerized polyester having an intrinsic viscosity of 0.67 is arranged in a sheath portion. In a raw filament having such a sheath-core structure, brought into contact with reeds or healds and scraped by receiving friction is the polymer of a peripheral surface portion, so that this is characterized in that the copolymer having a low glass transition point, which is difficult to be scraped against friction and fatigue, is arranged in the surface portion. However, both polymers arranged in the core and the sheath are too much different from each other in their characteristics, so that when the structure is fixed by heat treatment, only conditions taking into account the deformation of the sheath component polymer can be used. Accordingly, the structure of the core component is insufficiently fixed, or the draw ratio for expressing strength is forced to be set higher. As a result, the effect of filament scraping inhibition decreases to fail to obtain a screen gauge having sufficient performance. Further, this raw filament employs the polymers different from each other in compatibility between them, so that a separation phenomenon occurs in an adhesion interface of the polymers.
Furthermore, patent document 4 (JP-A-2003-213520), patent document 5 (JP-A-2003-213527), patent document 6 (JP-A-2003-213528) and patent document 7 (JP-A-2004-232182) proposes to use polyester polymers that are not copolymers as sheath components.
Of these, patent document 4 (JP-A-2003-213520) is characterized in that drawing is performed while irradiating infrared light to obtain a high-modulus monofilament having a breaking strength of 7.5 cN/dtex or more and a breaking elongation of 5 to 15%. However, an irradiation spot of the infrared light is extremely small, so that deflection of a running filament from the spot due to swing of the filament is liable to occur. It is therefore difficult to apply such a technique to industrial production. Further, the monofilament having a breaking elongation of 5 to 15% is difficult to absorb impact applied to a fabric, so that filament breakage at the time of weaving and filament breakage caused by fabric fatigue at the time of repeated use are liable to occur. Furthermore, this is also liable to contribute to filament breakage in a drawing process.
In addition, patent document 5 (JP-A-2003-213527) and patent document 6 (JP-A-2003-213528) are characterized in that fine inorganic metal particles are allowed to be contained in a sheath component polymer, thereby decreasing the friction resistance of a filament surface. This is caused by allowing the fine inorganic metal particles to deposit on the filament surface by bleedout thereof to roughen the filament surface. However, in the course of transferring a melt thereof, aggregated particles deposit to excessively roughen the filament surface in some cases. This can become the factor of scratching a metal surface of a reed to further increase defects such as filament scraping with time. It is apparent that the presence of excessive fine inorganic metal particles decreases mechanical characteristics of the resulting monofilament, specifically the elongation. It is also the same in such a sheath-core composite filament that pirn contraction caused by fiber structure strain internally existing as a filament structure by an increase in modulus becomes liable to occur. Here, the term “pirn contraction” as used herein means a stripe-like unevenness which can be seen by the situation when filaments of the inner layer portion of pirn is used as a weft. The pirn contraction is caused by tightening of winding in the inside of the pirn layer.
Moreover, patent document 7 (JP-A-2004-232182) proposes to subject this to a 2-10% relax treatment after drawing, thereby removing the fiber structure strain. However, when the monofilament is relaxed to such a large extent, an extremely large decrease in modulus at an intermediate elongation is induced, resulting in insufficient filament characteristics. When the draw ratio is further increased in order to compensate this, not only pirn contraction occurs but also the effect of inhibiting filament scraping according to the sheath-core composite structure is lost. Further, also in a drawing process, swing of a running filament becomes large under such large-relax conditions, which causes a factor of deteriorating process yield.
Further, patent document 8 (JP-A-2001-11730) proposes a method for obtaining a pseudo sheath-core type monofilament utilizing the difference in intrinsic viscosity, which is expressed by the difference in flow rate of a melt in the inside of a spinning pack. However, this method has a risk that the sheath-core ratio and the difference in intrinsic viscosity vary depending on the flow of the melt in the inside of the pack, and therefore lacks stability. Changes in the flow of the melt possibly occur using as a trigger changes in inner pressure balance of the pack caused by, for example, a clogged state of a filtration tank. Accordingly, a fear remains in stability for fluctuations with time of spinning, variations among spindles at the time when the spindles are increased and repeating reproducibility for each production lot.
[Patent Document 1] JP-A-55-16948
[Patent Document 2] JP-A-1-132829
[Patent Document 3] JP-A-2-289120
[Patent Document 4] JP-A-2003-213520
[Patent Document 5] JP-A-2003-213527
[Patent Document 6] JP-A-2003-213528
[Patent Document 7] JP-A-2004-232182
[Patent Document 8] JP-A-2001-11730